The present invention relates to interior permanent magnet motors and more particularly to an interior permanent magnet motor with a rotor comprising shifted laminations.
Permanent magnet brushless motors are widely used in industrial drives for high performance application because of their high torque density. One form of permanent magnet motors, interior permanent magnet (IPM) motors, are in relatively widely use due to the relative simplicity. For example, IPM motors commonly employ imbedded magnets that are relatively inexpensive, such as rectangular magnet blocks, embedded in their rotor cores. By embedding the magnets in the rotor core rather than affixing the magnets to an exterior surface of the rotor, magnet retention and manufacturing yield may both be improved without significantly impacting motor output characteristics relative to surface-mounted, permanent magnet (SPM) motors.
In high performance applications for IPM motors, cogging torque can become an important challenge, as well as torque ripple, vibration, and speed pulsations. Cogging torque, for example, may be caused by interactions between permanent magnets mounted on the rotor and slots that may be defined in an associated stator. To minimize cogging torque in an IPM motor can present significant challenges relative to SPM motors because IPM motors may include relatively smaller air gaps such that traditional techniques such as linear skewing or shaping of the magnets may be rendered impractical by the simplified rectangular shapes of the permanent magnets. As a result, attempts to minimize the cogging torque often entail added complexity and reduced power (i.e, torque) output for a given size of motor.
Current designs for interior permanent magnet motors (IMPs) seek to minimize cogging torque using a variety of techniques such as shaping of magnets and magnet poles, skewing of rotor and/or stator magnets, step-skewing of magnets, combining slots and pole, and placement of dummy notches in the stator teeth. Unfortunately, though, various harmonics can remain problematic. For example, in one recently encountered configuration, an 18th order harmonic emerged prominently and was attributed to a gap between adjacent segments of a stator in an IMP.
Accordingly, it is desirable to have improved systems and methods for reducing cogging torque in IPM motors. It is also desirable to have an improved IPM brushless motor providing reduced cogging torque characteristics across a full range of harmonic frequencies.